It has been a long term and continuously sought after goal to develop spark plugs that have longer service lives. At the present time, spark plugs have been developed for light to medium duty applications, such as in automobile engines, that have a service life of from about 30,000 to 100,000 miles. However, in heavy duty industrial applications, such as in large industrial engines often operating on natural or unrefined fuels at well heads or other remote locations, a typical service life for spark plugs, even with platinum or rubidium electrodes, is on the order of 1,500 to 2,500 hours of operation. Not only are the spark plugs for such engines initially costly, but they are often difficult to access and require that the engines be shut down several hours for cooling prior to removing old plugs and installing the new ones. Thus, changing spark plugs in these situations is not simply a matter of inconvenience, but one with significant economic disadvantages
One recent example of an attempt to improve the service life of spark plugs is disclosed in U.S. Pat. No. 4,743,793 issued May 10, 1988 to Akihiro Toya et al. Toya forms the electrical discharge tip of the spark plug electrode by boring a hole in an electrode, filling it with noble metal particles, and then ultrasonically bonding or welding the particles in the hole. U.S. Pat. No. 4,742,265 issued May 3, 1988 to Joseph M. Giachino et al discloses a spark plug having a center electrode formed of an alloy material containing aluminum and chromium. U.S. Pat. No. 4,427,915 issued Jan. 24, 1984 to Kanemitsu Nishio et al discloses a spark plug construction in which a portion of the center electrode is formed by sintering a matrix material containing a titanium compound and an electrical conductivity-imparting substance, such as platinum, with a noble metal such as gold, silver or ruthenium.
All of the above spark plug constructions are difficult to form and contain either costly noble metals or elements such as chromium that are environmentally disadvantageous.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a spark plug that has a significantly extended service life for use particularly in severe service cycle and heavy duty engine applications. Further it is desirable to have such a spark plug that does not contain costly, relatively rare, noble metal materials, or require the use of materials that are environmentally undesirable.
Importantly, it is desirable to have such a spark plug that is simple to construct and in which all of the electrode elements may be preformed prior to assembly. Moreover, the preformed electrode elements overcome the problems associated with difficult in situ processing steps such as powder compaction, sintering and subsequent shaping.